Since the first cases were recorded in 1981, acquired immunodeficiency syndrome (AIDS) and its causative agent, the human immunodeficiency virus (HIV), have taken an enormous toll around the world. It is estimated that, at the end of 2005, 38.6 million people were living with HIV/AIDS. This includes nearly 4.1 million who were infected in 2005 alone. In the last 25 years, more than 25 million people have died of AIDS, including 2.8 million in 2005.